Technologies regarding optical integrated circuits, in which optical components are integrated, such as transistor integrated circuits integrating electronic components have been developed. At present, optical circuits are constituted of optical components such as optical switches, wavelength filters and 3 dB couplers (optical couplers), which are connected together via optical waveguides such as optical fibers, wherein it is possible to significantly reduce the volume of optical circuits, power consumption and manufacturing cost if a plurality of optical components can be integrated on a small chip. Various optical integrated circuit technologies have been developed so far, Patent Document 1, for example, discloses an optical switch having a photonic crystal structure. The term “photonic crystal” (or “photonic crystalline”) is a general term regarding the structure undergoing periodical variations of refractive indexes of light.
Photonic crystals exhibit various unique optical features owing to periodically structured refractive-index profiles, wherein one exemplary feature is a photonic band gap (PBG). Photonic crystals are able to transmit light therethrough, but photonic crystals adequately undergoing large periodical variations of refractive indexes do not transmit light with a specific frequency band. The frequency band (or the wavelength range) of light transmitted through photonic crystals is called a photonic band. The frequency band of light not transmitted through photonic crystals is called a photonic band gap (PBG) implying that it occurs between photonic bands. Some photonic crystal structures may allow photonic band gaps to be overlapped with a plurality of frequency bands. Photonic bands split by photonic band gaps are referred to as a first band, a second band, and a third band in an ascending order of frequency.
When micro defects destroying periodically structured refractive-index profiles (or the periodicity of refractive indexes) appear in photonic crystals, light whose frequency matches frequency bands of photonic band gaps is confined in micro defects. In this case, light whose frequency matches the sizes of micro defects is solely confined in micro defects; hence, photonic crystals serve as resonators. For this reason, photonic crystals can be utilized as frequency-selective (or wavelength-selective) filters (or optical filters).
When a plurality of micro defects is consecutively arranged in lines to form line defects in photonic crystals, light with frequency bands of photonic band gaps is confined in line defects, allowing light to propagate along line defects. That is, it is possible to utilize photonic crystals exhibiting line defects as optical waveguides. Optical waveguides composed of photonic crystals containing line defects are called line-defect waveguides.
It is possible to constitute optical functional elements such as optical modulators and optical switches by using either optical filters or optical waveguides or by combining both of them. That is, it is possible to constitute optical circuits by forming and connecting optical functional elements in photonic crystals. For this reason, photonic crystals are expected to be the platform of optical integrated circuits.
Photonic crystal structures actually available as the platform of optical integrated circuits are subjected to the following limitations.
When the effects of photonic band gaps are adopted in a three-dimensional space consisting of X-axis, Y-axis and Z-axis, refractive-index profiles of photonic crystals need to be periodically structured in a three-dimensional manner. However, since photonic crystals exhibiting three-dimensionally structured refractive-index profiles are complex and produced at high manufacturing cost, photonic crystals exhibiting two-dimensionally structured refractive-index profiles (hereinafter, referred to as two-dimensional photonic crystals) have been frequently used. Specifically, actually used two-dimensional photonic crystals with finite thickness are formed on substrates, wherein refractive-index profiles indicate periodicity in a plane, but refractive-index profiles do not indicate periodicity in the thickness direction. In this case, the PBG effect does not contribute to confinement of light in the thickness direction of substrates, but the total reflection due to differences of refractive indexes achieve such confinement of light.
The optical features of two-dimensional photonic crystals with finite thickness do not perfectly match the optical features of two-dimensional photonic crystals with infinite thickness. However, when refractive-index profiles in the thickness direction of two-dimensional photonic crystals with finite thickness are made with mirror symmetry in light-propagating regions, the optical features of two-dimensional photonic crystals with finite thickness match the optical features of two-dimensional photonic crystals with infinite thickness. The operational prediction of two-dimensional photonic crystals with infinite thickness can be made far easier than the operational prediction of two-dimensional photonic crystals with finite thickness. For this reason, it is easy to design devices using two-dimensional photonic crystals exhibiting refractive-index profiles with mirror symmetry.
So far, various structures regarding two-dimensional photonic crystals with finite thickness have been developed, wherein pillar-type tetragonal photonic crystals exhibit a property (i.e. low group-velocity property) in which light-propagating speed is slowed down in a broad range of frequency bands. Generally speaking, it is possible to constitute optical circuits of predetermined functions with short waveguide lengths by use of waveguides of low light-propagating speed. Therefore, line-defect waveguides using pillar-type tetragonal photonic crystals are applicable to optical integrated circuits.
FIG. 12 is a perspective view showing the structure of a line-defect waveguide composed of a pillar-type tetragonal photonic crystal 100 with finite thickness. In the pillar-type tetragonal photonic crystal 100, cylinders 102, composed of high dielectric material with finite heights, and cylinders 103 whose diameters are smaller than those of the cylinders 102 are arranged in a tetragonal lattice array in a low dielectric material 101. The tetragonal lattice array of the cylinders 102, 103 is likened to the lattice array of atoms in silicones or crystals and used for optical applications; hence, those crystals are called “photonic crystals”. In this connection, the low dielectric material 101 and the cylinders 102, 103 are not necessarily composed of materials having crystal structures; hence, they can be composed of materials having amorphous structures.
In FIG. 12, the cylinders 102 are configured of completely periodic photonic crystals, whilst the cylinders 103 whose diameter is smaller than that of the cylinders 102 are regarded as defects that appear among the perfect crystals of the cylinders 102. To distinguish between the perfect crystals of the cylinders 102 and the defects of the cylinders 103, the former ones are referred to as “non-line-defect cylinders”, whilst the latter ones as “line-defect cylinder” in the following description. In this connection, the line-defect cylinders 103 do not actually develop defects by themselves.
In the pillar-type tetragonal photonic crystals 100, the line-defect cylinders 103 are arranged in lines, whilst the non-line-defect cylinders 102 are formed around the linear alignment of the line-defect cylinders 103 so as to form a line-defect waveguide. In the line-defect waveguide of the pillar-type tetragonal photonic crystal 100, the linear alignment of the line-defect cylinders 103 is deemed equivalent to the core of a waveguide of a total-reflection confinement type, whilst the array of the non-defect cylinders 102, on the opposite sides of the linear alignment of the line-defect cylinders 103, is deemed equivalent to the clad of the waveguide. Similar to the waveguide of the total-reflection confinement type, which implements the waveguide functionality with the core and the clad, the line-defect waveguide implements the waveguide functionality with the line-defect cylinders 103 and the surrounding non-line-defect cylinders 102 as well as the low dielectric material 101. The line-defect waveguide, when appropriately designed and produced, may serve as a single-mode waveguide indicating a basic mode alone.
As described above, wavelength filters are produced by forming micro defects in photonic crystals; in actuality, however, structures for inputting/outputting light need to be added to wavelength filters. For instance, Patent Document 2 discloses a wavelength filter, which is produced by arranging line-defect waveguides in proximate to micro resonators or by partially inserting micro resonators into line-defect waveguides. The structure in which micro resonators are arranged in proximity to line-defect waveguides may serve as an optical switch that performs ON/OFF switching of light.
Generally speaking, the wavelength filter of the micro resonator type exhibits Lorentz transmission characteristics, which suffers from a problem in that a small full width at half maximum matches a transmission bandwidth. That is, the micro-resonator waveguide allows light of resonance frequency to be solely transmitted therethrough, whilst the micro-resonator waveguide does not allow light of other frequencies, other than the resonance frequency, to be transmitted therethrough. By shifting the resonance frequency of a micro resonator in the high frequency side or the low frequency side, it is possible to switch over the ON state, which allows light of the original resonance frequency to be transmitted, and the OFF state, which does not allow light of the original resonance frequency to be transmitted or which causes reflection or radiation of light (i.e. propagation loss); but the wavelength involving the ON/OFF switching should be solely limited to the resonance frequency. For this reason, a method for achieving broadband transmission characteristics or flattop property has been developed in such a way that a plurality of micro resonators with the same structure is cascaded and optically coupled together. This makes it possible to simultaneously switch over two frequencies, which appear in proximity to the lower-limit frequency and the upper-limit frequency in a specific transmission band. Specifically, the transmission band of the micro-resonator waveguide is shifted in the low frequency side, thus implementing switchover from the first state, which allows for transmission at the first frequency proximate to the upper-limit frequency of a transmission band, but which reflects (or does not allow for transmission) at the second frequency proximate to the lower-limit frequency of the transmission band, to the second state which allows for transmission at the second frequency.